Actually, the N900 is listed as having Tri-Band UMTS support, meaning that even the Euro models will support AWS. So there shouldn't be any issues with using it in the US on T-Mobile.

However, Nokia only covers warranty for a device in the region/country that it was bought in. In other words, if you buy a Nokia phone from the UK and try to claim warranty in the US, you're out of luck. You would have to send the device to a Nokia service center in the UK to get a replacement or have it repaired. So there is that to consider.